Various apparatus for measuring substances, mainly used in clinical chemistry are already well known (See above all those of Yellow Spring Instrument Co. for measuring glucose and galactose, and those of Owens-Illinois (Kimble Division) for measuring urea). These types of apparatus use enzymatic electrodes, that is, electrochemical sensors on whose sensitive external membrane are deposited reactive substances such as enzymes.
By means of such apparatus, it is possible to measure indirectly other substances capable of reacting with the enzyme, forming a reaction product which can be detected by the electrode employed. By way of example, it is well known in documentation how it is possible to determine the concentration of glucose in the presence of glucos-oxidase enzyme, by measuring the oxygen used up in the reaction: ##STR1## In this case, a gas electrode, known as the Clark electrode, for measuring pO.sub.2, is used.
Another well known reaction is that for determining urea in the presence of urease enzymes, by measuring the ammonia NH.sub.3 or the NH.sub.4.sup.+ ion. Particularly, among the patents of the prior art the following are also to be mentioned. DS-Pat. No. 1,956,211 (Beckmann) uses electrochemical sensors, well known, suitable for direct differential measurement of SO.sub.2. The present invention provides instead an analyzer that measures a parameter which is indirectly correlated or can be correlated to another parameter. Therefore the Beckmann apparatus is not able to solve the problems recognized by the inventor, in that it offers the possibility of measuring only one parameter.
Moreover, the Beckman patent provides only one measurement phase, rather than a series of phases consisting of filling, measurement, advancement, reaction, return and final measuring of a fluid. A succession of phases like this assures much more significant and varied results. The subject of the U.S. Pat. No. 3,909,204 (J. D. Allen) is an apparatus for measuring N.sub.2 O and CO.sub.2 through a galvanic cell. Generally speaking, measurements made with these types of apparatus are not altogether reliable, as the final element (pO.sub.2, pNH.sub.4.sup.+ etc.) to be determined is also present in a sample under investigation in an unknown quantity, together with interfering substances for the sensor employed.